Introduction and Goals
This tutorial will begin our discussion of Kingdom Animalia. First, we will discuss what makes an organism an animal. Second, we will focus on some major innovations in the evolution of animals. Third, we will conclude with a discussion of animal development (from a single-cell zygote through a specialized embryo). The next four tutorials will focus on animal diversity (Figure 1). By the end of this tutorial you should have an understanding of:
- The general characteristics of all animals
- The origin and early evolution of animals
- The major branch points in the phylogeny of animal evolution
- The basic steps involved in animal development
- Summarize the general characteristics of animals
- Review the evolutionary history of animals and their relationship to protists
- Define tissues, symmetry, cephalization
- Describe the early stages of animal development, including tissue and gut formation
- Differentiate between radiata and bilateria, diploblasts and triploblasts
- Discuss the role of Hox genes in animal development
Figure 1. Overview of Animal Diversity and Body Plans (Click image to enlarge)
What are Animals?
We all agree that a cat is an animal, but what is it that makes it an animal? Why is a sponge an animal? Why are sponges and cats placed in the same kingdom? You might think that some corals look more like plants, and that sponges don't look like much of anything; at least, not like anything alive. Why are these diverse organisms (Fig. 2) all classified as animals?
There are some basic features that are found in all of the members of the kingdom Animalia. In general, animals are all motile, heterotrophic, and multicellular.
Figure 2. Animal Diversity in a Saltwater Aquarium. (Click image to enlarge)
Motility refers to the ability to move and/or affect motion in one's vicinity. For example, adult sponges don't move from place to place, but they can manipulate their cells to bring food to themselves. Also, while some animals are sessile as adults, all animals exhibit movement from place to place sometime in their development. For example, sponges and corals are motile as larvae (the very early stages of embryonic development) and sessile as adults (Fig. 3). A myriad of adaptations for motility are observed in animals, and some of these will be addressed in this tutorial. The image on the right depicts the development of a sponge.
Figure 3. Sponge Development. (Click image to enlarge)
Animals are ingestive heterotrophs, they ingest nutrients, bringing them into their bodies (Fig. 4). Unlike plants, which store their food as starch, animals store their food as glycogen.
Animals have muscle tissue and nervous tissue. These tissues can range from being simple (as seen in members of the radiata) to being highly complex (as seen in vertebrates). The coordination between muscle tissue and nervous tissue can result in very specialized movements (e.g., getting food, pursuing a mate, avoiding a predator).
Figure 4. Cheetahs preparing to ingest prey. (Click image to enlarge)
When Did Animals Arise?
Animals first appear in the fossil record approximately 700 million years ago, but some researchers think that animals might have evolved much earlier. Read about work in the link below that was conducted at Penn State University to determine when various taxa evolved. Be prepared to answer questions about their methods and conclusions in the tutorial quiz. Large Gene Study Questions Cambrian Explosion.
The Origin of Animals
The first animals likely originated from a colonial protist similar to choanoflagellates (Fig. 5). Choanoflagellates share some basic feature with animals. Molecular data also place them as the group of protists most closely related to animals. Animals first appear in the fossil record approximately 700 million years ago, but some researchers think that animals might have evolved much earlier. Read about work in the link below that was conducted at Penn State University to determine when various taxa evolved. Be prepared to answer questions about their methods and conclusions in the tutorial quiz.
Figure 5. Drawing of a Choanoflagellate. (Click image to enlarge)
Note that this colony of individual cells is anchored to a substrate, and that each flagellum sweeps food into a cell for ingestion. The cell can then transfer the food to other cells. While this organism shows little specialization, it is a very primitive form of multicellularity and ingestive heterotrophy.
Animal Evolution: The Big Picture
This figure illustrates the major branch points in the phylogeny (evolutionary history) of animal diversity. Study this figure; you will see it again.Each of these bifurcations (branch points) is a discrete dichotomous (division of an ancestral line into two equal diverging branches) point. For example, an organism is either a parazoan or a eumetazoan; eumetazoans are either radiata or bilateria (based upon their symmetry; we will discuss this later). Each bifurcation marks a character state that is thought to clearly separate the lineages at that point. The higher you go in the tree, the more derived (or more recently evolved) is each particular character.
Figure 1. Overview of Animal Diversity and Body Plans. (Click image to enlarge)
Parazoa Versus Eumetazoa
The first dichotomous branching point of the phylogenetic tree of Kingdom Animalia (Fig. 6) separates organisms that do not have true tissues from those with true tissues. A tissue is an aggregation of cells that performs a function. Parazoans lack true tissues, whereas eumetazoans have true tissues. There has been some debate about whether parazoans, the sponges, should even be considered animals; however, molecular data clearly place them at the based of the animal tree. Eumetazoans have distinct collections of cells that are arranged for specific purposes. Thus, the presence of true tissues is the first bifurcation in the animal phylogeny.
Figure 6. Overview of Animal Diversity and Body Plans.(Click image to enlarge)
Sometimes it can be easier to see similarities between groups at the larval stage. For example, sponge larva are free-swimming with flagellated cells (Fig. 7). As adults, they settle in one place and are no longer mobile; they do retain flagella on their cells (we will discuss the structure of sponges in more detail in Tutorial 19).
Figure 7. Sponge Development. (Click image to enlarge)
The Eumetazoans: Radial Versus Bilateral Symmetry
The second dichotomous branch point in the Animal phylogenetic tree (Fig. 8) separates the eumetazoans with radial symmetry (the radiata) from those with bilateral symmetry (the bilateria). In organisms that possess radial symmetry, multiple mirror image planes can be drawn across the organism’s center (think of slicing a pizza; you can slice at any point to divide the pizza into two equal halves). In organisms that possess bilateral symmetry, only one mirror plane can be drawn along a single axis. For example, humans have only one mirror plane, running from head to toe down the center of the body.
Another characteristic of the radiata is that they are diploblastic. Diploblastic organisms only have two embryonic tissue layers: endoderm and ectoderm. The endoderm gives rise to the lining of the digestive tract, and in higher animals, the liver and lungs. The ectoderm gives rise to the animal's outer covering and, in some phyla, the central nervous system. The bilateria are triploblastic and begin life with three distinct tissue layers: endoderm, mesoderm and ectoderm. The mesoderm gives rise to the muscles and most of the internal organs, including the heart and kidneys. We will discuss these tissue types more in the following tutorials. Note that these are embryonic distinctions; this does not mean that these organisms will end up with only two or three tissue types. Some animals that exhibit radial symmetry as adults (for example, the sea stars) have been placed in the bilateria. Why might this be the case? That is, do you think an adult morphology might differ significantly from the larval morphology of the same animal?
Additionally, the radiata tend to be sessile as adults, whereas the bilateria are motile. We will discuss this distinction later in the tutorials.
Figure 8. Overview of Animal Diversity and Body Plans. (Click image to enlarge)
Protostomes Versus Deuterostomes
The last major branch point of the phylogenetic tree of Kingdom Animalia distinguishes animals with a true coelom (body cavity) based on the fundamental aspects of their development (Fig 9; will will discuss the development of the body cavity in Tutorial 20). Protostome coelomates include the mollusks, annelids and arthropods, whereas deuterostome coelomates include the echinoderms and chordates.
One of the main differences between the two is the origin of the openings to their digestive tract. In protostomes the mouth develops first, whereas in deuterostomes the anus develops first. "Stoma" is Greek for "mouth," and "protos" and "deuteros" are "first" and "second," respectively. We will discuss more about the differences between protostomes and deuterostomes later. These two groups also have other differences in the development of their embryos.
Figure 9. Overview of Animal Diversity and Body Plans.(Click image to enlarge)
In the kingdom Animalia, organisms are primarily grouped according to their development. Early development is conserved across the members of this kingdom. This means that two animals can appear to be very similar when in an early developmental stage but be very different as adults.
Development is defined as those processes that are irreversible and occur from zygote formation to death. The word "irreversible" is key and limits the definition of development. Cellular differentiation is defined as how a cell diverges from its early morphology into a more specialized morphology, in an irreversible manner. Morphogenesis is how an organism's "shape" is acquired, and pattern formation describes how cells, tissues, and organs are arranged in an organism.
The development of a diploid organism begins with fertilization. Shortly after the gametes unite to form the single-cell zygote, cell division commences. These early cell divisions are called cleavage. Cell division is not a random process, and various patterns of cleavage are observed. In some cases (e.g., protostomes), a spiral pattern is observed in the first few cell divisions, whereas in other cases (e.g., deuterostomes), the blastomeres (early cells) assume a radial configuration.(Fig. 10A).
Besides having different types of cleavage, the embryonic cells of protostomes and deuterostomes have different potentials for future development. Animals that have a radial cleavage pattern (deuterostomes) have blastomeres that can potentially form all tissue types (i.e., their developmental fate is not predetermined); hence, this type of early development is termed indeterminate development. On the other hand, those animals with spiral patterns (protostomes) have blastomeres whose developmental fate is determined very early; hence, their development is termed determinate development.
Figure 10. Protostome Versus Deuterostome Development.(Click image to enlarge)
Totipotency is the capability of certain embryonic cells to form any type of adult cell. The early embryonic cells (blastomeres) of deuterostomes can be totipotent. Totipotency is required for two individuals to develop from a single zygote that is split in half (e.g., identical twins, also called monozygotic twins). Totipotency is typically lost during early embryo development, but scientists can reprogram some cells to become totipotent once again in the laboratory.
Figure 11. Dolly The Sheep - First Cloned Mammal. Dolly the sheep was cloned using the nucleus from a mammary (breast) cell that was rendered totipotent in the lab.
Stem Cells and Development
In the early deuterostome embryo, each cell has the potential to form a complete embryo, i.e., they are totipotent. As development proceeds some embryonic cells begin expressing new genes and at the same time they lose the ability to form all possible cell types, but they still retain the ability to differentiate into a subset of cells; these cells are said to be pluripotent. In the popular press, both totipotent and pluripotent cells can be referred to as stem cells. Understanding how stem cells maintain their totipotency (and how their derivatives lose this character) is an important area of research in developmental biology. Recently, scientists have discovered a method for reprogramming normal or “somatic” cells to become pluripotent, producing iPS cells, or induced pluripotent stem cells.
Medical researchers are also interested in this area as it may provide new treatment options for patients that have irreparable damage to certain tissues. However, ethical questions have arisen regarding the use of human embryos to obtain totipotent stem cells and consequently research, particularly in in this country, has limited the widespread acceptance of this research. In 2007, researchers reported on the production of pluripotent cells from highly differentiated human fibroblasts (a lightly differentiated cell) and this approach holds the promise to further advance stem cell research without the need of human embryos.
From Zygote to Induction
Figure 12 illustrates how the first collection of cells (blastomeres) form a hollow ball, the blastula. However, blastulae aren't always hollow spheres; they can be flattened (e.g., the blastulae of birds and mammals). From this hollow embryonic stage, cells start migrating into the interior (the blastocoel, which is the first cavity that forms) and simultaneously begin to differentiate. The cells that migrate inward express a different set of genes. Gastrulation, this movement of surface cells inward, is accompanied by induction (the process by which certain embryonic cells trigger the differentiation of other embryonic cells). During gastrulation, cells change their developmental pathway. The gastrula stage, in which tissue types arise, is only observed in the eumetazoans.
Figure 12. Early Embryonic Development Through Gastrulation. (Click to enlarge)
At the conclusion of gastrulation, the endoderm and ectoderm have been defined, and the mesoderm then forms (Fig. 10B). In addition, the archenteron and blastopore arise. The archenteron is a pouch of cells created by invagination during gastrulation, and the blastopore is the opening to the pouch. (In the simplest scenario, think of pushing into a lump of clay with your finger; the area that your finger tip forms (the pouch) represents the archenteron, and its opening represents the blastopore.) The blastopore's fate is determined by the type of organism (Fig. 10C). If the blastopore gives rise to a mouth, it is a protostome. If the blastopore gives rise to an anus, it is a deuterostome. The gut forms from the archenteron.
Hox genes determine the body pattern in animals
Hox genes are found in animals and determine the pattering of the body during the development of the embryo. These genes code for transcription factors, which control the expression of other genes. In animals, these genes are found in a linear sequence along the chromosome, starting with the genes that affect the structure at the head of the organism, and moving down the body (Fig.13).
Figure 13. Hox genes in the fruit fly, Drosophila melanogaster (Fig 13 top) and the mouse, Mus musculus (Fig 13 bottom). The color shows the location of the expression of a particular gene in the hox cluster (only one of the four mouse clusters is shown). http://www.nature.com/nrc/journal/v2/n10/box/nrc907_BX2.html (http://www.nature.com/nrc/journal/v2/n10/box/nrc907_BX2.html*)
These genes were first discovered in the fruit fly (an invertebrate model organism) which has one cluster of hox genes, and are now known to be in all animals. They are also well-studied in the mouse (a vertebrate model organism) which has four clusters of hox genes (humans also have four clusters). These additional clusters have taken on new functions, including determining the patterning of the limbs. Mutations in these genes can lead to a change in the postion of body parts, and the results include flies that have legs where their antennae should be!
This tutorial began our discussion of Kingdom Animalia. Animals have been around for over a billion years, and during this time they have diversified to a remarkable degree. The ancestor of all animals was most likely a colonial protist. In examining the animal phylogenetic tree, keep in mind that there are major dichotomous characters that occur at each branch point, including the presence of tissues, the number of tissue layers, and the symmetry of the organism. In many instances these characters are based on the development of animals.
Eumetazoan development (as with other sexually reproducing, multicellular eukaryotes) begins with fertilization. The zygote then undergoes a series of cell divisions to produce a mass of cells that has some hollow character (the parazoans do not follow this type of embryology). The first tissue to form is the blastoderm, and this young tissue surrounds the blastocoel. In the next step (gastrulation), cells migrate inward and begin to differentiate into endoderm. Thus, gastrulation marks the embryonic stage where additional tissue types form. In the case of diploblastic animals, there are only two tissue types (ectoderm and endoderm), whereas triploblastic animals have three tissue types (ectoderm, mesoderm, and endoderm). During gastrulation, the primitive gut also forms from the archenteron. Hox genes are found in all animals and determine the patterning of the body during development.
As we survey the various phyla of Kingdom Animalia in the next four tutorials, keep in mind the relationship between these major characters and animal development.
After reading this tutorial, you should have a working knowledge of the following terms:
There is no case study for this tutorial
Now that you have read this tutorial, go to ANGEL and complete the tutorial practice problems to test your understanding. Questions? Either send your instructor a message through ANGEL or attend an online office hour (the times are posted on ANGEL).